Air conditioning/gas usage


#1

Could someone out there answer this question that’s been nagging me, a world-class cheapskate, about whether a car uses more gas when you set the AC on high, and conversely, less when you set the temp higher. Is the question clear? Hope so, and I hope this isn’t a silly question. Thanks.


#2

Most AC’s (I think, mine anyway) control the temperature by mixing heated air from the heater with the cold air from the AC. The AC is running full blast all the time. Under these conditions (which I’m not sure apply to all autos) it obviously makes no difference where you set the temperature, the AC still puts the same load on the engine.

I have a suspicion that cars with an actual temperature control, that you set in degrees, function differently.


#3

Anytime the AC is in operation, it takes power from the engine. In order for the vehicle to perform the same as if the AC was off will require more fuel.

Tester


#4

Your question is unclear. The compressor makes the engine use more fuel. The compressor runs during AC and defrost modes. You can raise the air temp without using more fuel.

There has been a discussion for many years concerning how much extra fuel the car uses when the AC is running. During AC operation the windows are raised, which increases aerodynamic efficieny of the car. Driving with the windows down (and AC “off”) increases the aerodynamic drag on the car but eliminates the compressor drag on the engine. I’ve heard many times that running the AC with the windows up consumes less fuel. However, this is one of those “facts” that everyone knows but no one has ever read a test report.

A couple of months ago I heard on the radio that someone had done a test to compare AC windows up vs. no AC windows down. The report stated that the no AC windows down was significantly more fuel efficient than the alternate. I believe the cars were tested at speeds less than 45 mph. Comfort levels were not addressed. Speeds of 70 mph were not addressed.

As far as I’m know there has never been a worthwhile comparison tested and published.


#5

The controversy over the economy of using the A/C is based on a comparison of A/C on and windows up versus A/C off and windows down. Open windows increase aerodynamic drag on the vehicle, which may offset any alleged fuel savings by not running the compressor. If this is your question, then the answer is that there is still no agreement which is better.

If you are merely asking about fan speed or temperature settings, Bill above gave a good answer: no difference in fuel usage for either a cool or cold interior.


#6

You’re right but don’t even think about it too much.


#7

Short answer is it depends on the car. On some the A/C would run full time and it would blend outside and inside air to get it warmer. In others the compressor will cycle to maintain a temperature. I would guess only the fancy expensive jobs are going to cycle the compressor and they are in cars getting a high of 13 mpg anyway.


#8

Windows up with AC vs windows down with out AC. There most likely is a crossover point as your car’s speed increases. The horsepower needed to overcome air drag goes up by a factor of eight when you double the speed of a car. That’s four times the air resistance times twice the distance in a given unit of time. So, if you’re going down the freeway at 80 mph, it likely takes less power to drive with the windows closed and the AC on.
I just don’t know what speed the crossover happens at and it probably depends on the car model.


#9

Drag increases as the square of the speed increase, not power of 4.


#10

Joseph, you took the words right out of my mouth. Except that there are high end expensive cars that get good fuel mileage. And climate control systems rather than just heater and AC systems are making their way into lesser priced vehicles.

To the OP: what kind of vehicle are we talking here?


#11

Two squared IS four, so doubling your speed does quadruple the force needed to overcome the air resistance. Since power is force times speed, four times the force at twice the speed = eight times the power needed to overcome air resistance.


#12

If your driving a dinosaur the a/c really drags the engine, but if you are driving a modern car it does not make much of a difference. The reduction will not be noticed. If it is its better to arrive cool and fresh instead sweaty and on the verge of a heat related heart attach.


#13

First of all it’s fourth power not power of 4, which is something different, and a TV show.

Yes, doubling the speed quadruples the force needed to overcome the drag, which makes it a square function.

Power is amount of work done per second.


#14

There is also the issue of hearing loss. If you drive at highway speeds with the window down, you can suffer significant hearing loss in your left ear.


#15

WHAT? Just kidding. After a ride with the windows down people do get bad headaches from getting battered by the wind. Its true.


#16

Holy crap, I cannot believe you kids are bitchin about this. Nobody in their right mind gives a rats rear end about this while running down the highway with the cd cranked. And that includes you guys (be honest). Now put away those slide rules you just learned about and go get out your Guitar Player game.


#17

I think Edmunds tested the various gas-savings tips sometime back, when gas first headed towards $3 a gallon. The upshot was, AC made virtually no difference. The best gas-saving tip turned out to be (surprise) not driving like a maniac–no jackrabbit starts or hard stops. Second best was, don’t speed. Curiously, tire pressure didn’t make much difference.

Scrabbler


#18

see
http://www.edmunds.com/advice/fueleconomy/articles/106842/article.html
for the edmunds test


#19

Thanks for your very complete answer. I may have confused the issue in the manner I stated the question. I want to know if the engine requires more fuel if I make the AC colder, and conversely, if turn the AC “down” does it use less fuel?

Thanks again. I learned from your answer.


#20

I think I answered that question, but here is the answer again:
Most AC’s (I think, mine anyway) control the temperature by mixing heated air from the heater with the cold air from the AC. The AC is running full blast all the time. Under these conditions it obviously makes no difference where you set the temperature, the AC still puts the same load on the engine.